This method relates, in general, to CDMA (Code Division Multiple Access) spread spectrum receivers, and more specifically, to signal acquisition techniques by such receivers.
Spread spectrum communication in its basic form is a method of taking a data signal that is used to modulate a sinusoidal carrier and then spreading its bandwidth to a much larger value, e.g. in a Global Positioning System (GPS) application, effectively multiplying a single-frequency carrier by a highrate binary (xe2x88x921,1) pseudo-random noise (PRN) code sequence that is known to GPS users. Thus, the signal that is transmitted includes a data component, a PRN component, and a (sinusoidal) carrier component.
At the receiver, a synchronized replica of the transmitted PRN code is required to de-spread the data sequence. Initial synchronization, called acquisition, is followed by fine synchronization, which is called tracking.
The present invention relates only to acquisition, not tracking. Acquisition is the process by which, for the first time or after losing an acquired signal, the replica PRN code is synchronized (to within a small timing offset) with the code conveyed by the received signal and, in addition, the carrier frequency of the received signal is determined. Thus, the process of acquisition must determine any frequency-shifting of the received signal to allow for an accurate wipeoff of the carrier signal. The frequency-shifting includes Doppler-shifting as well as differences in frequencies because of clock inaccuracies. Satellite motion relative to the receiver causes a Doppler shift of the carrier frequency, which results in a modulation of a code component after carrier wipe-off in a GPS receiver. The replica code sequence must not only be time-aligned with the received code sequence, but also modulated to compensate for the frequency shifting to fully eliminate the PRN sequence and leave behind only the data conveyed by the received signal. Thus the acquisition is a two-dimensional search in code phase and frequency domain.
For GPS signals the search interval in the frequency domain can be as large as 12 kHz, i.e. the search interval can be as much as +/xe2x88x926 kHz about a nominal carrier frequency. In addition, the code phase can be any possible value of code phase, due to uncertainties in position of the satellite and time of transmission of the received signal. A PRN code is typically 1023 chips (bits of code, as opposed to bits of data) in length (before repeating). Thus, the acquisition module of a receiver must search a 12 kHz wide interval with 1023xc3x97ks different code phases, where ks denotes the number of samples per chip.
Ordinary GPS receivers designed only for operation with unobstructed satellites search for the frequency shift with a granularity of 1 kHz. Thus, such receivers must search 12xc3x97ksxc3x971023 different code/frequency combinations.
A GPS receiver designed for indoor operation must have an operating mode with equivalent noise bandwidth of approximately 50 Hz in the acquisition stage. Even with a 50 Hz bandwidth though, some post-detection filtering must be performed as well as further refining of the frequency for reliable tracking. The granularity of 50 Hz requires that the receiver search 240xc3x97ks xc3x971023 different code/frequency combinations, and makes the sequential search a time-consuming task, motivating the use of parallel and fast search methods.
What is needed is a way of performing the code/frequency search in a way that is computationally efficient and also accurate.
Accordingly, the present invention provides an apparatus, a system, and a corresponding method for acquiring a spread spectrum signal that includes a carrier component at a carrier frequency, a code component, and a data component, where the acquiring includes matching the phase of a replica of the code component to the phase of the received code component and also determining the carrier frequency including any shifting of the carrier frequency. The apparatus includes: a multi-section matched filter having a plurality of sections, the multi-section matched filter being responsive to a signal derived from the spread spectrum signal by a sequence of processing steps including mixing the spread spectrum signal with a first frequency, for providing successive section outputs for each section, each section output of each section including elements corresponding to a different replica code phase; and a compensated acquisition module, responsive to each section output of each of the sections, for providing a sequence of second frequency indicators, each second frequency indicator corresponding to a different frequency offset in a group of frequency offsets from the first frequency, and each section output having elements each of which corresponds to a particular value of replica code phase, wherein each second frequency indicator is based on a combining, phase element-wise, of the section outputs using compensating factors that depend on the corresponding frequency offset.
In a further aspect of the invention, the combining using compensating factors is a combining of the section outputs for only one code period.
In a further aspect of the invention, the combining using compensating factors is a coherent combining of more than one code period of the section outputs, the term coherent combining being used to indicate a combining of terms in which the phase or sign of the terms is taken into account.
In another, further aspect of the invention, the apparatus also includes an analysis module, responsive to the sequence of second frequency indicators, for providing a replica code phase and a corrected carrier frequency.
In still another, further aspect of the invention, the combining using compensating factors is coherent combining, and in a further aspect, the compensated acquisition module includes: a coherent combining module, responsive to the section outputs, for combining the section outputs a plurality of times, each combination using different compensating factors based on a frequency offset selected from the group of frequency offsets, to provide one or more successive groups of sequences of compensated sums; and a non-coherent combining module, responsive to the one or more groups of sequences of compensated sums, for accumulating, frequency offset-wise, the magnitudes of each element of the one or more successive groups of compensated sums.
In still another, further aspect of the invention, the multi-section matched filter and the compensated acquisition module are co-located in a receiver.
In yet still another, further aspect of the invention, the apparatus is a distributed system in that the compensated acquisition module includes computational elements located in a facility separate from the facility or apparatus that hosts the multi-section matched filter.